1. Field
The system and method of the present invention pertains to the field of recycling waste materials comprising cellulosic fiber and thermoplastic resin; more particularly, an improved method for heating and maintaining the temperature of dies while processing recycled combustible materials into products capable of generating high heat outputs.
2. Background
An increasing number of consumer products are made from thermoplastic resin such as, for example, adhesive liners and medical gowns. Some consumer products, such as, for example, disposable diapers, are primarily made up of thermoplastic resin and cellulosic fiber wherein the thermoplastic material provides a moisture-proof lining on the outside of the diaper and the cellulosic fiber provides the bulky absorbent media on the inside. The cellulosic fiber holds and retains all moisture, while the thermoplastic material ensures that there is no external leakage.
When products such as diapers, adhesive liners, hygiene pads and the like are manufactured, a certain amount of waste is inevitable, resulting in so-called “pre-consumer waste.” In addition, many of these products are disposable in nature and, as a result, are used just once and thrown away resulting in “post-consumer waste.” The ultimate disposal of pre-consumer and post-consumer waste typically involves transporting it to the local landfill. Environmentalists abhor this type of disposal as being wasteful both in the manufacture and disposal of these products. For example, the manufacture of disposable diapers requires forest products to obtain the necessary cellulose and the disposal of the diapers utilizes valuable landfill space. Moreover, the U.S. Environmental Protection Agency (EPA) has placed increase restrictions on landfill requirements. For example, the EPA has recently enforced the requirement of double lining landfills for disposal of paper mill sludge. Consequently, there has been a dramatic increase in cost for establishing new landfills that comply with EPA requirements for paper mill by-products.
In addition to the increased reluctance to use forest products and increased restrictions in landfill requirements, there has also be an increase in demand for new sources of energy. Combustible products made from cellulosic fibers and thermoplastic resins offer a higher BTU output and provide a clean-burning alternative to conventional fuels. However, use of available cellulosic waste as a fuel source has achieved only limited acceptance to date. One reason for this is the relatively low heating value of cellulose as compared to, for example, coal. For example, cellulosic fibers alone can have a heating value of less than 7,000 BTU's per pound, while coal generally has heating value in excess of 9,000 BTU's per pound. Another problem is that many consumer products have substantial tear-resistant properties because the polymers are highly cross-linked or otherwise heavily processed, making these products exceptionally difficult to shred or extrude.
Methods and systems for processing materials consisting substantially of thermoplastic resin and cellulosic fiber into combustible materials are well known in the art. Typically, these processes typically consist of placing the materials in slow-speed, high-torque shredders where the material is shredded to a consistent size and then moved by a conveyor line to a “cuber,” or extrusion machine, where fuel cubes are extruded under pressure. However, there are a number of problems that arise with this process.
For example, in recent years, many companies have made significant advances in improving the tear-resistant properties of thermoplastic materials. These highly tear-resistant materials, by their very nature, are exceptionally difficult to process using conventional means. For example, if these materials are processed through normal shredder devices, the shredder will quickly become bound-up and, in many cases, cease operating. Moreover, because the materials are combustible by nature, they have a propensity for catching fire if exposed to high heat or friction, such as during processing. As a result, if the operator is successful in maintaining the operation of the shredder, the friction involved in processing these materials creates an extreme fire hazard. There is a need, therefore, for an improved method for processing recycled combustible materials into products capable of generating high heat outputs.
Currently, cuber die temperatures are monitored with metal-to-metal contact thermocouples, each configured to monitor one cuber die quadrant. The disadvantages of this technique are twofold. First, if a heater element in a first die in which a thermocouple is mounted, or a die adjacent to the first die, fails, the reading from the thermocouple no longer reflects the average temperature of the quadrant. Those skilled in the art will recognize this to be a common occurrence and one that is not addressed by any prior heating system. Second, once the above situation occurs, the quadrant in question begins to go into thermal runaway. Because the controller is being fed a low temperature signal, it increases the on cycle times of the remaining heaters in an attempt to compensate. This results in excessive heater temperature, which induces additional heater element failures. In prior systems, no automatic detection systems existed so, therefore, if this situation is not detected by the operator, the quadrant in question eventually goes into a chain reaction style runaway situation, which often results in damage to electronics as well as spontaneous ignition of the cubes upon discharge from the dies.
Accordingly, it is on object of the present invention to provide an improved method for monitoring and maintaining die temperatures during the processing of thermoplastic resins and cellulosic materials.
Another object of the present invention is to mitigate system runaway due to erroneous readings obtained by a contact style sensors.
Another object of the present invention is prevent system underloading due to erroneous readings obtained by a contact style sensors.
Another object of the present invention is to enhance the overall cube quality and uniformity through consistent, predictable die temperatures.
Yet another object of the present invention is to reduce the risk of a cuber fire due to temperatures of the dies exceeding preset limits.